Optical measuring apparatus for use on machines

ABSTRACT

Optical apparatus for use with a machine having parts movable along three mutually orthogonal axes. The apparatus uses a single laser to direct a main beam along a first one of the axes and a square deflector mounted on a movable part of the machine to direct the beam orthogonally into a second direction without any angular errors due to pitch or yaw of the square deflector. Thus only roll of the square deflector need be measured. Further square deflectors are used to deflect the beam around other axes of the machine whereby the co-ordinates of a measuring probe can be accurately checked against the machine scale readings. Various methods of measuring roll are described including generating side beams from the main beam and directing them in anti-parallel direction to the main beam onto detectors which detect the deviation of the anti-parallel beams due to roll of the square deflector.

The present invention relates to optical measuring apparatus forchecking the accuracy of a machine, for example, a co-ordinate measuringmachine or a machine tool.

Co-ordinate measuring machines consist of a base on which a workpiecemay be mounted, a mechanical structure carrying a measuring probe, andmeans for producing relative movement between the base and the measuringprobe in three mutually orthogonal directions whereby the measuringprobe can be accurately located in all three directions known as the x,y and z directions. Scales attached to the base and the mechanicalstructure, and which extend in the x, y and z directions, are read byopto-electronic read heads to provide information whereby a computer cancalculate the x, y and z co-ordinates of points on a workpiecepositioned on the base which are sequentially contacted by the measuringprobe.

Similarly machine tools require to position a cutting tool along threeco-ordinate axes to perform cutting operations on a workpiece, or toposition a measuring probe for measuring the workpiece after a cuttingoperation as part of the production process. Movements of the relativelymovable machine parts are read by scales and read heads in known manner.

The accuracy of the measurements of the workpiece obtained using ameasuring machine depends on how closely the readings taken by the scaleread head represent the position in space of the measuring probe. Thescale readings, however, take no account of relative pitching, rolling,yawing and lateral movements of the parts of the mechanical structureduring said relative movement which arise due to slackness in themachine slideways, or due to physical bending and twisting of the partsof the structure. In order to minimise these pitch, roll and yaw andother movements, the machine structure has to be made relatively massiveand the slideway along which the relative movements of the structuretake place have to be machined extremely accurately, use being made ofair bearings on the slideway to provide ease of movement between therelatively massive structures and the base.

Similar inaccuracies occur during relative movements of parts of machinetools which affect the accuracy of the cutting operations and themeasurements made by a measuring probe fitted in the tool for measuringworkpieces after machining.

As an alternative to the expense of building greater accuracy into suchmachines, increasing use is being made of optical techniques formeasuring the position of the probe independently of the scale readings.By this means the errors due to pitch, roll, yaw and lateral movementsin the measurements of the machine framework can be identified andeliminated by making corrections to the measurement readings in themachine software.

One example of such independent measuring apparatus is described in U.S.Pat. No. 4,261,107 in which a plurality of laser systems is provided ona machine to measure the yaw, pitch and roll components of the machinemovement in addition to the x, y and z movements. This system has theproblem that it requires multiple lasers and beam benders on each of thethree axes to measure all of the errors.

Other examples of optical techniques for checking errors in measuringmachines and machine tools are shown in U.S. Pat. Nos. 3,661,463, and4,276,698.

U.S. Pat. No. 3,661,463 describes a system in which a single laser beamis deflected sequentially along all three axes of a machine, thusreducing the number of lasers, but this system has no capability formeasuring roll, pitch or yaw of the moving parts during their movement,and thus does not correct the machine reading for these errors.

U.S. Pat. No. 4,276,698 describes a system in which although pitch, yawand roll of the parts of a machine are acknowledged, the errors due tomany of these movements are discounted as insignificant. In largemachines however, these errors may not be insignificant. Also, in thedescribed method of making optical measurements, the laser is in somecases mounted on movable parts of the machine. The measurements given bysuch a system however, are themselves inaccurate, because of the pitch,roll and yaw of the laser device on the movable axis, unlessmeasurements are taken on one axis at a time.

These problems are reduced with the optical apparatus in accordance withthe invention as claimed in the appended claims whereby in a machinehaving parts movable relatively to each other along mutually orthogonaldirections, there is provided at least one square deflector positionedon one of the movable parts of the machine so that when a beam of lightfrom a light beam generator is directed onto it along a first one ofsaid directions, it deflects the beam into a second one of saiddirections, and means for measuring rolling movements of the squaredeflector about the axis of the light beam.

Throughout this specification the term "square deflector" is to beunderstood as defining an optical device which deflects a light beamtravelling in a first direction through an angle (nominally a rightangle) which may deviate about the axis of the incident beam due torolling movement of the device about the same axis, but which remains ata fixed angle irrespective of pitching or yawing movements of the deviceabout the axis of the incident beam. Devices known as `pentacubes` or`pentaprisms` have this property.

One advantage of the optical apparatus of the invention is that the beamgenerator, which is preferably a laser beam generator, may be mounted onfixed structure of the machine so that inaccuracies due to its ownpitch, roll and yaw movements are eliminated.

Another advantage of the optical apparatus of the invention is that byusing a square deflector on the movable part of the machine, angulardeviations of the deflected light beam from the second direction due topitch and yaw of the deflector are elimated. Thus by continuouslymeasuring the distance of the deflector from an origin, together withthe straightness of the incident beam and roll of the deflector, boththe vector of the deflected beam and its point of origin in the squaredeflector can be accurately determined at all times during the movementof the movable member. Thus the square deflector acts as a point oforigin in the second direction, of a beam, the direction of which isknown with reasonable accuracy.

By providing such optical apparatus in, or parallel to, all of theorthogonal co-ordinate axes of the machine, using square deflectors todeflect incident beams from one direction into the next one, angulardeflections of the beams due to pitch and yaw of the machine parts inall axes can be eliminated thus reducing the number of measurementsneeded, and simplifying the optical measuring apparatus.

Pitching and yawing movements of the deflector give rise to smalllateral or fore and aft displacements of the deflected beam which can beof the same order of magnitude as errors in straightness of the beamsand can, if desired, be taken into account by a straightness measuringdevice at the end of the deflected beam.

In a preferred embodiment of the invention a single laser light sourcegenerates one beam which is deflected through the square deflectors totraverse optical paths in, or parallel to, all co-ordinate directions ofthe machine. The beam is also split to provide other beams which areused to generate the measurements for roll and lateral deviation(straightness) of the beams traversing the optical paths. Thus inaccordance with the invention the means for directing a beam onto asquare deflector may in fact be a square deflector in a previous opticalpath.

However, where so many beam splitters are required in the opticalmeasuring apparatus that a single laser generator would beinappropriate, additional laser generators may be provided to injectadditional optical power into the optical paths. This may requireadditional measurements of pitch, roll and yaw of the additional lightbeam generators if the accuracy of the measuring apparatus so requiresunless a suitable fixed structure can be found for the laser.

Some of the roll errors may be of second order importance depending onthe design of the measuring machine and the accuracy required of it.Thus some of the errors may be ignored or taken into account by errormapping the movements of one or more parts of the machine. Thus by usingthe optical apparatus of the present invention which eliminates some ofthe errors, and by choosing which errors should be measured directly andwhich should be covered by static or dynamic error mapping, a machinemeasuring system can be produced which gives high accuracy withrelatively less components whereby the cost and complication of thesystem may be reduced.

In accordance with another aspect of the invention means for measuringrolling movement of an object about an axis during movement of theobject along the axis comprises means for directing a light beam alongsaid axis, a pair of beam splitters each fixed to the object andpositioned to receive a portion of the light beam and arranged toproduce a pair of side beams, two reflectors each fixed to the objectand positioned to receive the side beams and to reflect the side beamsinto directions parallel to or anti-parallel to said light beam ontodetectors positioned to detect deviation of the two reflected side beamscaused by rolling motion of the object about said axis.

Where high accuracy is required of the measurement, both of the two beamsplitters and both of the two reflectors are preferably squaredeflectors.

The invention will now be more particularly described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic view of the layout of a co-ordinate measuringmachine incorporating optical measuring apparatus of the presentinvention.

FIG. 2 is a diagram defining roll, pitch and yaw.

FIG. 3a is a general diagram showing the part of the optical apparatusand beam path in the y-z plane for producing measurements in the y axis.

FIG. 3b is an enlarged exploded view in the x-y plane of the opticalapparatus of FIG. 3a showing the optical system S1 in greater detail.

FIG. 4a is an exploded view in the x-z plane of the apparatus showingthe optical devices comprising the optical system S2 in greater detail.

FIG. 4b is a side view of the optical system S2 shown in FIG. 4a.

FIG. 5 is an exploded view in the y-z plane of the optical devicescomprising optical system S3, and

FIG. 6 is an exploded view in the y-z plane of the optical devicescomprising the optical system S4.

FIG. 7 is an exploded view in the y-z plane of an alternative opticalsystem to S4.

Referring now to the drawings there is shown in FIG. 1 a co-ordinatemeasuring machine 10 having a base 11, a mechanical structure 12 movableof tracks 13 on the base and which extend in the y co-ordinatedirection. The supports for the structure 12 on the tracks arepreferably air bearings, known per se, but may be any other convenientsupport, for example slides or wheels, which fit closely in the tracks13. It is to be understood that as an alternative construction the framemay be rigidly attached to earth while the base moves on the tracks.

The structure 12 consists of two pillars 15, 16 supporting a cross beam17 extending in the x co-ordinate direction, which in turn provides arelatively rigid support for a spindle 18 which extends in the zco-ordinate direction. The spindle 18 carries a measuring head 19 havinga measuring tip or probe 20 for contacting a workpiece for determiningthe co-ordinates of points on the surface thereof. The spindle 18 issupported by a carriage 25 in tracks (not shown) on the cross beam 17 byany convenient means, preferably air bearings, for movement along thebeam to provide positioning of the spindle in the x co-ordinatedirection, and the spindle 18 may be raised and lowered in conventionalmanner in a bearing (not shown) to provide positioning of the measuringtip or probe in the z co-ordinate direction. Hence the sum of all of themovements enables the measurement tip or probe to be located anywherewithin the measuring volume of the machine. Scales (not shown) providedon the base 11, the beam 17 and the spindle 18, combined with read headson the pillar 15 and the carriage 25 provide, in a manner known per se,readings of the positions of the relatively movable parts of thestructure, and in known manner these readings are fed to a computer 29which calculates and displays the readings as x, y and z co-ordinates ofthe measuring tip or probe.

The scale readings give a nominal reading of the x, y and z co-ordinatesof the measuring tip 20, but due to the possibilities of each of themoving parts pitching, rolling or yawing about its axis during itsmovement, or being physically displaced e.g. due to slackness in thetracks, there is a degree of uncertainty about the precise positioningof the measuring tip 20. The present invention provides an opticalmeasuring apparatus which can either give an accurate measurement of theposition of the measuring tip 20 independently of the readings of thescales, or can provide error correction signals for each of the scalesseparately.

For the purposes of this specification the terms roll, pitch and yaw inrespect of any one of the movable parts are defined with reference toFIG. 2 as follows: assuming the direction of movement is along the xaxis,

(i) arrow A illustrates roll which is the rotation of the part about thex axis,

(ii) arrow B illustrates pitch which is the up and down motion, i.e.movement in the direction of the z axis (which is equivalent to rotationabout the y axis), and,

(iii) arrow C illustrates yaw which is the lateral displacement i.e.movement in the direction of the y axis (which is equivalent to rotationabout the z axis).

The requirements of the apparatus are to provide a number of mainoptical beams extending in the direction of three principal orthogonalaxes x, y and z of the machine, and measurement beams with which roll ofthe moving parts of the machine about those principal axes, togetherwith straightness of the movement and the distances moved can bedetermined.

The optical systems may take many forms and provide the apparatus withthe capability of measuring all of the deviations of the movablestructure, or just those felt to produce the greatest error in theposition of the stylus tip as determined by the scale readings. Also theoptical systems may be the same on each axis, or may differ. Forexample, different considerations apply where fixed or movable structurehas to be used for mounting parts of the system, e.g. the laser ordetectors, or where the structure of the machine is such that on anygiven axes some of the deviations are so insignificant that they may beignored.

The apparatus shown in FIG. 1 is a diagrammatic composite illustrationof a machine in which the optical arrangements for different axes aredeliberately chosen to illustrate different aspects of the inventionwhich cover different possible considerations. The best solution for anygiven machine will however depend on whether or not the accuracy of themeasurement obtained can be compromised to allow for a reduction in thenumber of components, or in the performance and cost of the components.

FIG. 1 depicts a laser beam generator 30 and a plurality of opticaldevices (S1 to S4) for receiving and directing one or more laser beamsaround the machine structure. A plurality of photo-diode detectors areshown positioned at various points on the machine structure to providemeasurements of beam positions. For example two detectors D22, D23 aremounted on a convenient fixed structure indicated by two pillars 27 and28 on the y axis of the machine for supporting the detectors.

The detectors are arranged to produce electrical signals from the beamsimpinging on them, and the electrical signals are fed to the computer 29and may be used as correction signals to correct the scale readings toproduce more accurate x, y and z co-ordinates for the probe tip 20. Thedetectors may be conventional split photo-diodes, i.e. a pair ofphoto-diodes arranged side-by-side, or, for measurements in more thanone direction, a quadrature detector may be used i.e. four photo-diodesin a square configuration. Alternatively, photo-diode arrays orposition-sensitive detectors (PsDs) or interferometric straightnessmeasuring devices may be used.

The functions of the various optical devices will now be described usingbeam path diagrams for each of the separate axes with reference to FIGS.3 to 7.

In FIG. 3a there is shown a laser beam generator 30 mounted on pillar 27and arranged to produce a main beam B1 of laser light which is directedin the direction of the y axis of the machine. The main beam B1 ispassed through an interferometer 40 to an optical system S1 mounted onthe movable pillar 15. Optical system S1 is arranged to produce a secondmain beam B2 at right angles to main beam B1 in the y-z plane, andtargetted on an optical system S2 (FIG. 4) also on pillar 15.

In FIG. 3b it can be seen that optical system S1 has several opticaldevices in axial succession along beam B1 consisting of aretro-reflector R1, three pentacubes P1A, P1B and P1C and a photo-diodedetector D1. All of the optical devices have on their rear reflectingfacets as required, prisms which provide beam splitting surfaces forsplitting beam B1 to provide both the main beam B2 (from pentacube P1C),and the required number of side beams (from pentacubes P1A and P1B) toperform the measuring function.

Two side beams SB1A and SB1B are directed from pentacubes P1A and P1Brespectively onto further pentacubes P1D and P1E. These pentacubes inturn produce measuring beams MB11 and MB12 respectively directed back inanti-parallelism to beam B1 to detectors DO1 and DO2 respectivelymounted on the pillar 27 to determine the roll of the optical system S1about the beam B1.

The retro-reflector R1 produces a further measuring beam MB13 (FIG. 3a)which is also directed back in anti-parallelism to bean B1, and belowit, to the interferometer 40 mounted on pillar 27. Using theinterference between beam MB13 and a reference beam generated from beamB1 in the interferometer, the distance of the retro-reflector R1 fromthe interferometer may be accurately measured. Thus the distance of theorigin of beam B2 in pentacube P1C can be calculated since in practicethe retro-reflector and all of the pentacubes in optical system S1 arerigidly connected together.

A photo-diode quadrature detector D1 is provided at the end of theoptical system to measure the straightness of the movement of pillar 15.Optical system S1 thus gives very accurate information about theposition of the origin of beam B2 and deflects it into the direction zwithout transmitting any of the angular errors arising due to pitch andyaw of the optical system S1 as the pillar 15 moves. Thus the vector ofbeam B2 is known to a least first order accuracy.

If it becomes necessary to reduce the number of beams split from beam B1and to reduce the number of optical components, the retro-reflector andinterferometer may be deleted and a further pentacube or beam splittermay be put into the optical device S1 to direct a beam towards the readhead (or an extension thereof) to impinge on a photo-diode on the readhead thus giving a comparison of the actual position of beam B2 with thereading of the scale produced by the read head.

As an alternative to the above described method of measuring roll ofoptical system S1 deviation of the beam B2 from the vertical may bemeasured by an electronic inclinometer (known per se) attached to systemS1 whereby the photo-diode detectors (DO1 and DO2) may be eliminated.

Optical system S2, which can be seen in exploded form in FIGS. 4a and4b, is arranged on top of the pillar 15 so as to be able to direct abeam along the x-axis to the spindle 18. The system includes in axialsuccession along main beam B2 five pentacubes P2A, P2B, P2C, P2D andP2E, which have on their rear reflecting facets as required, prisms toprovide beam splitting surfaces both for the production of the main beamB5 for the next axis, and the various measuring beams.

A photo-diode quadrature detector D21 is connected to pentacube P2E.This detector receives the main beam B2 and detects any straightnessdeviation (i.e. x and y lateral translation) of the beam which may becaused for example by pitch or yaw of the pillar 15.

Since the direction of main beam B2 is known from optical system S1other than for such relatively second order lateral deviations, theidentification of the straightness errors now defines the direction ofmain beam B2 completely. Since the systems S1 and S2 are rigidlyconnected to the pillar 15 the distance between pentacubes P1C and P2Dand hence the position of P2D can be pre-determined very accurately.

It can be seen that pentacube P2A and P2B produce two measuring beamsMB21 and MB22 which are orthogonal to beam B2 and directed in oppositedirections towards detectors D22 and D23. These beams will moverespectively in opposite directions into and out of the y-z plane (i.e.in the x direction) if the pillar 15 twists, thus producing roll ofoptical system S2 with respect to optical system S1. Any such movementwill be detected by the two detectors D22 and D23, and calculation ofthe algebraic difference of the two detectors readings in the xdirection will provide a measure of the roll of optical system S2relative to optical system S1 about the axis of beam B2. Pentacube P2Dproduces a main beam B3 directed in the direction x towards the opticalsystem S3. Use of a pentacube ensures that angular errors due to pitchand yaw of optical system S2 about main beam B2 are eliminated. Thus theonly error required to be measured to define the vector of main beam B3is the roll of system S2 about the axis beam B2. As described above,this is determined from detectors D22 and D23.

Thus the origin of the vector of main beam B3 and its direction alongthe x-direction can now be accurately defined because even second orderstraightness errors in beam B3 due to pitch and yaw of system S2 can bedetermined from detector D21.

The above description of systems S1 and S2 shows a possible measurementsystem when the detectors D22 and D23 can be placed on fixed structure.This is only practical when there are no major translational movementstransverse to the line joining the optical systems and the detectors,otherwise the detectors would have to be at least as long as themovements.

Considering the problems of measuring roll of the next system S3 aboutthe axis of the main beam B3 (i.e. the x-axis) it can be seen that anybeams generated which are anti-parallel to main beam B3 for such rollmeasurement can only be directed to detectors supported on the top ofpillar 15. Alternatively since pillar 15 has a fixed relationship withpillar 16, beams may be generated which are parallel to the beam B3 anddirected at detectors supported on the top of pillar 16. Since pillar 15or 16 may undergo pitching, rolling and yawing movements relative tooptical system S3 there will be a degree of uncertainty as to whetherthe signals produced by the detectors are due to these movements or toroll of the system S3 which is what is required to be measured.

To increase the degree of certainty pentacubes P2C and P2E are providedto produce side beams SB2A and SB2B directed at the rear faces of twodouble sided split photo-diode detectors D24 and D25 to provide a datumagainst which the anti-parallel beams MB32 and MB33 (see also FIG. 5)can be compared (see below).

Although side beams SB2A and SB2B are shown having considerable lengthit must be borne in mind that in practice in the example described aboveall of the optical devices shown will be rigidly fixed together and thelengths of beams SB2A and SB2B will be short. Thus although pentacubeshave been illustrated for producing the side beams, thus eliminatingangular errors in these side beams due to any pitch and yaw movements ofpentacubes P2C and P2E of the optical system S2, in practice thesemovements will result in negligible displacements of these side beam ondetectors D24 and D25 so that simple beam splitters could replacepentacubes P2C and P2E.

Finally, as part of optical system S2 an interferometer 44 is includedin the path of main beam B3 so that the distance between the pentacubeP2D and the optical system can be measured.

Turning now to FIG. 5 it can be seen that optical system S3 has severaloptical devices in axial succession along the axis of beam B3,consisting of a retro-reflector R3 and four pentacubes P3A, P3B, P3C andP3D, with a photo-diode quadrature detector D31 attached to the finalpentacube P3D.

All of the optical devices have on their rear reflecting facets asrequired, prisms to provide beam splitting surfaces for producing boththe next main beam B4 (from pentacube P3B) and the required side andmeasuring beams. The functions of the optical devices are:

Retro-reflector R3 returns a portion of main beam B3 as measuring beamMB31 back in anti-parallelism to main beam B3 to the interferometer 44of optical system S2 to measure the distance of the retro-reflector fromoptical system S2.

Pentacube P3A produces a side beam SB31 which is directed to the rearface of a photo-diode detector D32. This beam is used as a datum againstwhich a measuring beam MB43 from optical system S4 is compared becausein the same manner as in described with reference to optical system S2,there is a degree of uncertainty about the position of the detector dueto pitching, rolling and yawing movement of the optical system S3 onwhich it is mounted. Pentacube P3B produces main beam B4 directed alongthe z axis alongside the spindle 18 and through a further pentacube P3E.By use of a pentacube pitch and yaw movements of the optical system S3due to carriage movements in the direction of main beam B3 do notprovide any angular error in the direction of main beam B4 which, willremain orthogonal to main beam B3 in the z axis. Thus measurement ofroll of the optical system is all that is needed to determine the vectorof main beam B4, apart from straightness errors.

Pentacube P3E provides a measuring beam MB32 directed back inanti-parallelism to main beam B3 to the front face of detector D24 inoptical system S2.

Pentacube P3C produces side beam SB32 which is directed at a furtherpentacube P3F, which in like manner to pentacube P3E, produces ameasuring beam MB33 which is directed in anti-parallelism to the mainbeam B3 to the front face of the detector D25 of optical system S2. Thedifference in the readings of any movement of the measuring beams MB32and MB33 on the front faces of the detectors D24 and D25 in the ydirection compared with, and corrected by, the readings from the rearfaces of the two detectors, gives an accurate measurement of roll of theoptical system S3.

Pentacube P3D produces a side beam SB33 directed at the rear face of aphoto-electric diode detector D33, and is used as described above inrelation to detectors D24 and D25 of optical system S2 to locate thedetector D33. Detector D31 measures lateral movements in the y, zdirections of the carriage 25.

Thus, once again, the origin of main beam B4 is located from thedistance measurement given by the interferometer 44 of system S2 and theknowledge of the vector of main beam B3, and can be corrected for rollerrors as measured by detectors D24 and D25, the position of main beamB4 can be accurately calculated along its path to optical system S4.

One final component may be required in optical system S3 and that is aninterferometer 46 through which main beam B4 is directed and which isarranged to receive a return measuring beam MB41 from the optical systemS4 to measure the distance of optical system S4 from optical system S3.

Referring now to FIG. 6 it can be seen that optical system S4 has manyof the optical devices of optical system S3 with the notable exceptionof a pentacube for deflecting main beam B4 into yet another orthogonaldirection as clearly this is not necessary at the bottom end of thespindly 18. Optical system S4 will only be briefly described therefore.

It consists of three optical devices in axial succession along the axisof main beam B4 and provided with beam splitting surfaces from whichportions of the main beam B4 are taken to provide the required side andmeasuring beams. The optical system includes a retro-reflector R4 whichprovides a return measuring beam MB41 directed at the interferometer 46of the optical system S3, and first and second pentacubes P4A and P4Barranged to produce side beams SB41 and SB42. The side beams SB41 andSB42 are directed at further pentacubes P4C and P4D which produce twomeasuring beams MB42 and MB43 directed to impinge on the front faces ofdetectors D33 and D32 respectively to provide the measurement of theroll of the spindle 18. A photo-diode quadrature detector D41 isprovided attached to pentacube P4B

Thus it can be seen using the invention in accordance with the firststated aspect that a laser beam can be deflected and traced around aco-ordinate measuring machine, or machine tool, by using a squarereflector to deflect the beam from axis to axis and by measuring theroll of the square deflector about the axis of the incident beam. Theexample described above shows how this may be achieved on all axes andshows various ways in which the different problems of roll measurementmay be overcome. Clearly not all axes need to be measured since theconstruction of some types of machines may ensure that roll on some acesis not a significant error. However, the invention will simplify thetask of tracing a beam around a machine even if it is only used on oneaxis.

Where possible the number of beam deflections should be minimisedbecause of the power limitations of the laser. For example, the firstdeflection through optical system S1 may be eliminated by mounting thelaser 30 on the top of pillar 27 and measuring the roll of the opticalsystem S2 by a method similar to that previously described for system S1for example.

In another modification the interferometer 46 and retro-reflector R4could be replaced as shown in FIG. 7 by a pair of pentacubes P5A and P5Bpositioned respectively in beams B4 and MB43 and connected rigidlytogether in optical system S3 where pentacube P5A deflects a portion ofthe main beam B4 as a beam MB44 across at right angles to pentacube P5Bestablishing a reference length, and interfering beam MB44 with beamMB43 in pentacube P5B. The resulting interference pattern can bedetected in a photo-diode detector mount to receive the combinedinterfering beam.

In the described system, the detector used for measuring roll have allbeen located using reference beams targetted to an additional detectionmounted on the rear surface of the roll detectors. An equally validscheme is for the reference beams and the roll measuring beams to beincident from the same direction. Such an example may occur if it wasdecided to mount the detectors which measure the roll of optical systemS3 about B3 on top of pillar 16, rather than as an integral part of theoptical system S2.

In this case (see FIG. 5) measurement beam MB32 and MB33 are nowparallel to the incident beam B3. To achieve this, all that is requiredis to rotate pentacubes P3E and P3F through 180 degrees about beams B4and SB32 respectively.

Although a system has been described for measuring roll using parallelon anti-parallel beams and split photo-diodes, it is understood thatroll can alternatively be measured interferometrically usingstraightness interferometers as described in U.S. Pat. No. 3,790,284,whereby each side beam for measuring roll is split into two beams. Withreference to FIGS. 4 and 5 and measuring the roll of optic S3 about beam3, the beam splitting prisms may be attached to pentacubes P3E and P3F,and the straightness reflectors correctly orientated would replace thefront detectors of D24 and D25. Beams SB2A and SB2B are still requiredso that the straightness reflector can be located if desired forimproved accuracy. In the example the displacement interferometricsignal can be obtained by inserting a beam splitting surface ontopentacubes P3E and P3F and using the appropriate detector.

All of the examples described above have illustrated the use of a laseras the light beam generator which enables the use of interferometricmovement techniques for length and straightness measurements. Whereother means for measuring length and straightness are used, however, acollimated light beam as opposed to a coherent laser light beam may beused.

Other modifications are also possible. Although FIG. 3b shows the beamB1, MB11 and MB12 to be all co-planar, it will be appreciated that thisneed not necessarily be the case. There is no need for beam B1 to beco-planar with beams MB11 and MB12. Also it is shown in the figure thatMB11 and B12 are anti-parallel to beam B1. Provided, the detectors canbe properly located, there is no reason why beams MB11 and MB12 cannotbe parallel to beam B1, or one parallel and the other anti-parallel.Similar considerations apply in optical systems S2, S3 and S4.

Furthermore, the pentacubes P1A, P1B could if desired be replaced bysimple beam splitters, and P1D and P1E could be replaced by simplereflectors, if pitch errors are negligible. Also, the reason for havinga pair of beam splitters P1A, P1B, a pair of reflectors P1D, P1E and apair of detectors DO1, DO2, is to counteract the effects of pitch, yawand straightness errors. If some or all of these are negligible, then itis possible to build a satisfactory system with only one appropriatelymounted beam splitter, reflector and detector. Again, similarconsiderations apply in optical systems S2, S3 and S4.

We claim:
 1. Optical measuring apparatus for use on a machine having afirst part movable along a first axis and supported on a second partmovable along a second axis orthogonal to the first axis, the apparatuscomprising in combination:means for directing a collimated light beamtowards the second machine part in a direction substantially parallel tothe second axis, a square deflector mountable on the second machine partin the path of the collimated light beam and for deflecting at least aportion of said light beam to produce a deflected beam directedsubstantially parallel to the first axis measuring beam generating meansmountable on the first machine part, for receiving the deflected lightbeam from the square deflector and for producing therefrom at least onemeasuring beam, and, detector means including at least one firstdetector positionable in the path of a said measuring beam in anorientation such as to produce a signal indicative of transversedisplacement of the measuring beam caused by rolling movement of thefirst machine part about the first axis.
 2. Optical measuring apparatusas claimed in claim 1 and wherein the square deflector is a pentacube orpentaprism.
 3. Optical apparatus as claimed in claim 1 and wherein themeans for directing a collimated light beam towards the second machinepart is a laser beam generator.
 4. Optical apparatus as claimed in claim1 and wherein the means for directing a collimated light beam towardsthe second machine part is a further square deflector.
 5. Opticalapparatus as claimed in claim 1 and wherein the measuring beamgenerating means produces two measuring beams.
 6. Optical apparatus asclaimed in claim 1 and wherein the measuring beam generating meanscomprises a first optical device for producing a side beam and a secondoptical device for receiving the side beam and for deflecting it into adirection substantially parallel to said first axis, the detector meansbeing positionable on the second machine part for receiving thedeflected side beam.
 7. Optical apparatus as claimed in claim 6 andwherein the first optical device is a square deflector.
 8. Opticalapparatus as claimed in claim 6 and wherein both the first and secondoptical devices are square deflectors.
 9. Optical apparatus as claimedin claim 6 and wherein further means are provided mountable on thesecond machine part in the path of the collimated light beam forproducing one or more further side beams directed towards the firstmachine part, said detector means including at least one furtherdetector rigidly connected respectively to the or each first detectorand positional be such that the or each further side beam impinges on afurther detector to provide therefrom a signal indicative of transversedisplacement of the detector relative to the collimated light beam. 10.Optical apparatus as claimed in claim 9 and wherein said further meanscomprise at least one square deflector.
 11. Optical apparatus as claimedin claim 9 and wherein said further means produce two further sidebeams.
 12. Optical apparatus for measuring rolling movement of a firstpart of a machine about an axis, said rolling movement being caused bymovement of the first machine part relative to a second part of themachine along said axis, the apparatus comprising in combination:meansmountable on the second machine part for directing a collimated lightbeam substantially parallel to said axis towards the first machine part,means mountable on the first machine part for receiving the collimatedlight beam and for producing therefrom at least one side beam, meansmountable on the first machine part for deflecting the or each side beamto produce a respective measuring beam or beams directed towards thesecond part of the machine in a direction substantially parallel to saidaxis, and detector means mountable on the second part of the machine forreceiving said measuring beam or beams and oriented to produce a signalindicative of displacement of the measuring beam or beams transverse tosaid axis and caused by rolling movement of the first part of themachine about said axis.
 13. Optical apparatus as claimed in claim 12and wherein the means for producing the or each side beam comprises atleast one square deflector.
 14. Optical apparatus as claimed in claim 12and wherein the means for producing said at least one side beam producestwo side beams.
 15. Optical apparatus as claimed in claim 12 and whereinthe means for deflecting the or each side beam comprises at least onesquare deflector.
 16. A method of measuring rolling movement of a firstmachine part about a first axis along which it is movable relative to asecond machine part wherein the first machine part is supported on thesecond part which is itself movable along a second axis orthogonal tothe first axis, comprising the steps of:directing a collimated lightbeam towards the second machine part in a direction substantiallyparallel to the second axis, positioning a square deflector on thesecond machine part in a manner such that it can receive at least aportion of the collimated light beam and deflect it towards the firstmachine part in a direction substantially parallel to the first axis,receiving the deflected portion of the collimated light beam on anoptical device on the first machine part and producing therefrom atleast one measuring beam, positioning detector means including at leastone first detector in the path of the or each measuring beam in such amanner as to produce a signal indicative of transverse displacement ofthe measuring beam caused by rolling movement of the first machine partabout the first axis.
 17. The method as claimed in claim 16 andcomprising the further steps of:providing on the detector means at leastone further detector respectively rigidly connected to the or each firstdetector, positioning the detector means on the second machine part,producing from the collimated light beam at least one further side beamdirected towards the first machine part and impinging on one of thefurther detectors to provide therefrom a signal indicative of transversedisplacement of the detector means relative to the collimated lightbeam, and directing the or each measuring beam from the optical deviceon the first machine part to impinge on a respective one of the firstdetectors.
 18. A method of measuring rolling movement of a first machinepart about an axis of the machine caused by movement of the firstmachine part relative to a second machine part along said axis, themethod comprising the steps of:directing a collimated light beam alongsaid axis towards the first machine part from directing means mounted onthe second machine part, receiving the collimated light beam on thefirst machine part and generating therefrom at least one side beam,deflecting the or each side beam by means of a deflector mounted on thefirst machine part to produce a respective measuring beam or beamsdirected towards the second machine part in a direction substantiallyparallel to the axis, receiving the or each measuring beam on a detectormounted on the second machine part and positioned to produce therefrom asignal indicative of displacement of the measuring beam or beamstransverse to said axis and caused by rolling movement of the first partof the machine about said axis.